In a MOSCAP, e.g. silicon-insulator-silicon capacitor (SISCAP), phase adjuster, charge is accumulated or depleted at the interface of a thin insulating region between two conductive regions. The metal conductive region is typically a p+ or n+ doped poly-silicon region, that behaves like a metal. The change in effective index of the waveguide material is driven by a modulation of the carrier density in an electrically active waveguide. In SISCAP Mach-Zehnder modulators, comprising a pair of waveguide arms, a MOSCAP phase adjuster causes changes to the effective index of the waveguide material resulting in a differential change in phase of the light propagating in the arms, which can be used to modulate an optical signal.
In conventional SISCAP modulators, such as the one disclosed in U.S. Pat. No. 7,657,130, issued Feb. 2, 2010, to Shastri et al, the conductive and insulating regions are superposed layers, extending horizontally parallel to the substrate. Unfortunately, horizontally layered SISCAP phase adjusters do not provide the same flexibility in waveguide design as lateral SICAP devices, and therefore not provide as good a mode confinement and resulting phase shift performance
An object of the present invention is to overcome the shortcomings of the prior art by providing a laterally disposed MOSCAP phase adjuster, and a method of fabricating a MOSCAP phase adjuster.